Co-precipitation-strengthened nickel base alloys and method for producing same

ABSTRACT

NICKEL BASE ALLOYS OF 15-22% CR, 3-12% MO; AT LEAST ONE OF THE GROUP CONSISTING OF 6-10% CB AND 6-9% V, AND 0.03-0.15% C, BALANCE NICKEL AND RESIDUAL IMPURITIES, ARE STRENGTHENED BY CO-PRECIPITATING DISPERSED MONOCARBIDES HAVING PARTICLE SIZES OF LESS THAN 250 ANGSTROMS AND AN INTERMETALLIC PHASE BY SEQUENTIALLY: (1) SOLUTION HEAT TREATING THE ALLOY AT A TEMPERATURE OF 22502300*F. (2) QUENCHING AND (3) COLD-WORKING IN REPETITIVE CYCLES. THE MATERIAL IS THEREAFTER FINALLY ANNEALED, QUENCHED AND AGED TO PRODUCE THE CO-PRECIPITATIONSTRENGTHENED STRUCTURE.

March 14, 1972 P. s. KOTVAL CO-PRECIPITATIONSTRENGTHENED NICKEL BASEALLOYS AND METHOD FOR PRODUCING SAME 2 Sheets-Sheet 1 Filed June 20,1969 Fine Particles of MC carbide distributed (in association withplanar lattice defects) throughout the alloy matrix.

A M Precipate (approx. l0,000 X) Grain boundary with M 0 carbideStrengthening MC carbide finely dispersed.

I 'Primary particle of MC carbide A M precipitate (approx. 8, 000 X amml M Tb m Mm QR Wh/ O n m$% u n & m

Y h 58 e P March 1972 P. s. KOTVAL 3,64

CO-PHBCIPITATION-STRENGTHENED NICKEL BASE ALLOYS AND METHOD FORPRODUCING SAME Filed June 20, 1969 2 Sheets-Sheet 2 As Cast" lngot orHot-worked Billet Solution heat treatment at 2250-2300F to partiallydissolve the"primary' MC carbides and to homogenize Quench Cold WorkSolution heat treatment at 2250-2300F Quench Cold Work Solution annealat 2250-2300F Quench Age at temperatures in the range of H00- I450F forbetween 30-lOOhours MC +A M Coprecipitation Strengthened alloy INVENTORPeshotan Sohrab Korval ATTORN EY United States Patent 3,649,379C0-PREClPITATION-STRENGTHENED NICKEL BASE ALLOYS AND METHOD FOR PRODUC-ING SAME Peshotan Sohrab Kotval, Indianapolis, Inc., assignor to CabotCorporation, Boston, Mass. Filed June 20, 1969, Ser. No. 834,942 Int.Cl. C22c 19/00; C21d 7/00 US. Cl. 148-12.7 Claims ABSTRACT OF THEDISCLOSURE Nickel base alloys of -22% Cr, 312% M0; at least one of thegroup consisting of 6l0% Ta, 69% Cb and 69% V, and 0.030.l5% C, balancenickel and residual impurities are strengthened by co-precipitatingdispersed monocarbides having particle sizes of less than 250 angstromsand an intermetallic phase by sequentially: (1) solution heat treatingthe alloy at a temperature of 2250- 2300 F. (2) quenching and (3)cold-working in repetitive cycles. The material is thereafter finallyannealed, quenched and aged to produce the co-precipitationstrengthenedstructure.

This invention relates to precipitation strengthened nickel base alloysand more particularly to alloys of this type which are strengthend bythe co-precipitation of monocarbides and an intermetallic phase. Theinvention also relates to a method for producing co-precipitationstrengthened nickel base alloys.

Precipitation strengthened nickel base alloys have heretofore been basedupon the formation of the well known aluminum-titanium rich phase knownas gamma prime. This strengthening mechanism requires the presence ofaluminium and titanium in an amount of as much as 8% by weight.

It is the main object of this invention to provide a nickel base alloywhich is strengthened by the co-precipitation of two new phases.

Another object is to provide a precipitation strengthened nickel basealloy which does not require the presence of aluminum and titaniumtherein.

A further object is to provide a nickel base alloy which has beenstrengthened by the co-precipitation of an A M phase and an MCmonocarbide phase wherein A consists essentially of nickel and Mconsists essentially of one or more elements selected from the groupconsisting of Ta, Cb and V, wherein C is carbon.

Yet another object is to provide a method for producing aco-precipitation strengthened nickel base alloy having an A M phase anda MC monocarbide phase as defined above.

Other objects and advantages will be apparent to those skilled in theart after consideration is given to the following specification,drawings and appended claims.

In the drawings:

FIGS. 1 and 2 are schematic representations of the microstructure of thealloys of the present invention; and FIG. 3 is a diagrammatic processflow chart outlining the steps required for making the alloys of theinvention.

The present invention is based upon two discoveries which areutilized-in combination to produce new nickel base alloys as well as amethod for producing same.

The first discovery resides in the fact that it is possible toprecipitate an intermetalic strengthening phase of the .A M type asdefined above from a nickel base matrix. It should be understood thatthis precipitation phase is distinct from the above mentioned grammaprime phase achieved through the use of aluminum and titanium incombination. It is known that the gamma prime phase has 3,649,379Patented Mar. 14, 1972 a crystal structure of the ordered face centeredcubic (FCC) type. In marked contrast to this, the A M phase producedwithin the alloys of the present invention is noncubic in crystalstructure. Although the precise crystal structure of theprecipitation-strengthening phase in the alloys of the invention has notbeen conclusively established, the best crystallographic evidencesuggests that it is a tetragonal structure wherein the axes of the unitcell of the crystal structure are of unequal length thus the crystalstructure is clearly not of the cubic type. It has also been found thatthis phase may be precipitated without the presence of aluminum andtitanium in the alloy composition.

The second discovery resides in the fact that it is possible toprecipitate a well dispersed distriubtion of monocarbides havingparticle sizes of less than 250 angstroms in the alloy. Most prior artwrought nicket base alloys contain sufiicient carbon so as to formmonocarbides of a simple face centered cubic structure whenevermonocarbide forming elements such as V, Ta and Cb are present. It isimportant to note that these monocarbides are usually in the form ofundissolved primary particles present in the alloy matrix but which donot effectively strengthen the same. Typically, primary monocarbideparticles have particle diameters of between 10 and 20 microns in nickelbase alloys. By partially dissolving the abovementioned primarymonocarbides, as will be explained further hereinafter, it is possibleto precipitate a dispersion of monocarbides having a particle size ofless than 250 angstroms.

This invention contemplates the co-precipitation of the abovementionedintermetallic and the monocarbide phases concurrently.

According to the invention, nickel-base alloy compositions are providedhaving a new microstructure. This microstructure is comprised ofintermetallic-cum-monocarbide co-precipitated phases. The interrnetallicphase may be defined as A M wherein A consists essentially of Nickel andM consists essentially of one or more elements selected from the groupconsisting of Ta, Cb and V. The monocarbide phase may be defined as MCwherein M consists essentially of one or more elements selected from thegroup consisting of Ta, Cb and V, and C is carbon, the MC monocarbidesbeing in the form of fine particles of less than 250 angstroms size. Thestructure would also contain primary monocarbides of 5 microns or lessin size.

The chemical composition of the abovementioned alloys of the inventionconsists essentially of (by weight): 15- 22% Cr, 312% M0, at least oneof the group consisting of 7-10% Ta, 6-9% Cb and 69% V, the totalTa-i-Cb-i-V content not exceeding 10%. The alloy must also contain0.030.15% carbon with the balance being nickel and residual impurities.The impurities should have not more than 7% Fe and not more than 8% Co.

The ability to produce the abovementioned nickel base alloy results fromthe discovery that by controlling the level of certain impurities in thenickel base alloy, (i.e. Fe) it is possible to solution heat treat thealloy at temperatures of about 2280 F. without causing liquation andmelting. Although, ideally, higher temperatures would normally berequired to dissolve monocaribdes, it is possible to achieve partialdissolution thereof by progressively subjecting the material to analternating sequence of high temperature solution heat treatments atabout 2280 F. and quenching together with controlled cold working steps.

Chromium is required in the alloy within the range disclosed above toprovide strengthening and corrosion resistance. Less than 15% Cr yieldsan alloy with minimal corrosion resistance; over 22% Cr yields an alloywith reduced ductility. Molybdenum is present in the alloy within theranges shown above to provide further solution strengthening andcorrosion resistance as required. Molybdenum is preferred in the alloy,although tungsten may replace molybdenum in whole or in part. Tungstenmay be present in greater quantities than molybdenum, i.e., up to amaximum of 16 percent, but preferably about 12 percent nominally. Carbonmust be present in the alloy within the range about 0.03 to 0.15% byweight to promote the formation of carbides in the alloy. Less than0.03% carbon is insufficient to produce the carbides while over 0.15%carbon tends to yield a more brittle alloy. Alloys containing over 0.15%carbon are also more difiicult to work. The alloy system of thisinvention must also contain at least one of the group includingtantalum, colnmbium, and vanadium. At least one of these elements mustbe present in the alloy together with carbon to provide the metalmonocarbides that are precipitated in the nickel-chromium (molybdenum)matrix. The presence of these precipitated carbides in addition to theaforementioned A M intermetallic phase together with the criticalproccessing steps that promote the controlled precipitation are theheart of the present invention. Each element in this group must bepresent within the ranges stated above when that element is theprincipal carbide and intermetallic former. The total content oftantalum-lcolumbium+vanadium in the alloy system must not exceed byweight. Iron may be present up to a maximum of 7%. This requirementresults from the fact that the higher iron content tends to lower themelting point of the alloy thereby prohibiting the higher solution heattreatment temperature required to cause the dissolution of the primarymonocarbides so that subsequent reprecipitation can be achieved as willbe hereinafter described. Cobalt may be present up to a maximum of 8%.Higher cobalt contents tend to lower the matrix stacking fault energyand thereby cause impediments in the nucleation of the dislocationreaction which is a necessary precondition for the desired morphology ofmonocarbide precipitation.

As aforementioned, the use of aluminum and titanium in the alloycompositions of the invention is not required. It is desirable that thealuminum plus titanium content should be as low as possible, andpreferably less than 1.4% If the aluminum plus titanium level is low, itwill be possible to produce a good quality alloy with an air meltingpractice.

Boron, silicon, manganese, magnesium, and copper up to a total of about2.5% may be present in the alloy within the ranges known in the art tobe effective to enhance certain characteristics associated with theseelements; i.e. the deoxidation step, casting fluidity, ductility and thelike.

The balance of the alloy is nickel and adventitious impurities generallyknown to be present in this class of alloys.

Referring now to the drawings, FIG. 1 is a threedimensionalrepresentation of the microstructure as would be observed in a thin foilsample using transmission elec- -prirnary MC monocarbides P. The actualsize of the primary MC monocarbides P shown is about l-2 microns. Theunique structure further has dispersed within it, fine precipitates ofMC monocarbides K which are associated with sheets of planar latticedefects. The precipitates K- are of about 250 angstroms or less in sizeand being coherent with the matrix strengthen it. The unique structurestill further has, uniformly dispersed within the matrix, an A Mintermetallic phase which is coherent therewith and which in combinationwith the precipitated MC monocarbide phase greatly strengthens theentire alloy system.

Referring now to FIG. 3, the alloys of the invention can be made byproviding an alloy material with 15-22% Cr, 312% M0, at least one memberselected from the group consisting of 710% Ta, 6-9% Cb and 69% V, thetotal of Ta+Cb+V not exceeding 10%, and 0.03-O.15% C, the balance beingnickel and residual impurities said impurities consisting of not morethan 7% Fe and not more than 8% Co, said alloy being in a form capableof being cold-worked.

The material is solution heat-treated at a temperature within the range22502300 F. for a period of about 24 hours to homogenize the matrix andto at least partially dissolve the primary monocarbide particles andthereby cause the monocarbide-formingelement and the carbon to be putinto solid solution in the alloy matrix. Following thissolution-heat-treatment the material is water-quenched and cold worked.Until the final desired dimensions of the product are achieved, theabovementioned sequence of solution-heat-treatment followed by quenchingand coldworking is repeated. Thereafter, the cold-worked material isannealed for a period not exceeding one hour within the solutionheat-treatment temperature range of 2250-2300 F. After annealing, thematerial is water-quenched to about ambient temperature and isthereafter aged at a temperature within the range of 1100-1450 F. for aperiod of 24 to hours. As aresult of this aging step, both the A Mintermetallic phase and the fine (less than 250 angstroms) monocarbidephase are precipitated uniformly throughout the matrix. The occurrenceof these two coherent phases in combination is the desiredstrengthening-mechanism of this invention. The invention will now beillustrated by the following examples:

EXAMPLE I A 5 lb. heat of material consisting of (by wt.): 20.34% Cr,8.90% Mo, 0.11% C, 8.58% Ta, 4.8% Fe, 0.08% Cb, 0.23% Al, 0.01% Ti, thebalance being nickel was electron-beam melted and cast into a 1.5" roundbar. This bar was solution heat treated at a temperature of 2282 F. fora period of 24 hours and water-quenched. Thereafter, the material wassequentially cold-worked, solution-heattreated at 2282 F. and quenched.After the process was repeated five times, the resultant 0.004" sheetproduced was solution heat treated (annealed) for a period of 1 hour at2282 F. and quenched. Thereafter, the material was cut into variousspecimens, each of which was aged at a different temperature and time.The range of aging temperatures was ll001450 F. and the range of agingtimes was 0.5 hours to 1500 hours. In all cases, thin foil transmissionelectron microscopy anddiffraction revealed that the requisitestructure, as depicted in FIGS. 1 and 2, and consisting of a uniformintragranular distribution of the two co-precipitated coherent phasesi.e. an A M intermetallic phase and a MC monocarbide ph'asewas obtained.The examination further showeda complete absence of a face centeredcubic gamma prime phase.

EXAMPLE II A 50 1b. heat of material consisting of (by weight): 20.47%Cr, 5.41% Mo, 0.089% C, 9.82% Ta, 5.59% Fe and 5.02% Co, the balancebeing nickel was vacuum induction melted and cast. After homogenizationat a temperature of about 2282 F. over a period of 24 hours the ingotwas forged to one inch square bar. .This asforged material wasthereafter solution heat treated..(ho mogenized) at 2282 F. for a periodof 24 hours and water-quenched. Thereafter, the material wassequentially cold-worked, solution heat-treated at 2282". F. andquenched until a form of 0.004" sheet was attained. The material wasthereafter further processed as in Example 1. Electron microscopy and.diffraction analysis revealed that the structure shown in FIGS. 1 and 2and described in Example I was attained. As in Example I, there was acomplete absence of any gamma prime formation present in the alloyproduced.

While the invention has been described in connection with specific alloycompositions and heat treatment steps, it should be understood thatminor variations may be made therein Without departing from the spiritand scope of the invention.

What is claimed is:

1. A nickelbase alloy consisting essentially by weight of 15 to 22% Cr,3 to 12% Mo, 0.03 to 0.15% C and at least one of the group consisting of7 to 10% Ta, 6 to 9% Cb and 6 to 9% V, the total of said group notexceeding about 10%, and with the balance being nickel and residualimpurities, provided that the alloy is essentially free of aluminum andtitanium and contains not more than 7% Fe and not more than 8% Co, saidalloy having both a coherent, non-cubic intermetallic phase representedby general formula A M and a coherent facecentered cubic monocarbidephase represented by the general formula MC wherein A consistsessentially of Ni, M is a metal from the group consisting of Ta, Cb andV and C is carbon, said monocarbide phase being present both as primaryparticles of up to about microns in size and as fine particles of lessthan 250 angstroms in size which are co-precipitated with the saidintermetallic A M phase.

2. A nickel base alloy as claimed in claim 1 wherein W is substitutedfor at least part of the Mo content.

3. A nickel-base alloy as claimed in claim 1 which is essentially freeof conventional precipitated gamma prime phase.

4. A nickel-base alloy as claimed in claim 1 wherein the Cr content isabout 20%, the Ta content is about 10%, the C content is about 0.09%,and the Mo content is about 5 to 8%, and the balance consistsessentially of nickel and residual impurities.

5. A nickel-base alloy as claimed in claim 1 wherein M is principally Tawith some Cb associated therewith.

6. A method for producing a nickel-base alloy strengthened byco-precipitation of two coherent phases, namely a non-cubicintermetallic phase A M and a FCC monocarbide phase MC, wherein Aconsists essentially of Ni, C is carbon, and M is a metal from the groupconsisting of Ta, Cb and V, said MC monocarbide phase being dispersed asfine particles of less than 250 angstroms in size in addition to primaryparticles of up to about 5 microns in size, comprising the steps of:

(a) providing in a form capable of being cold-Worked an alloy materialconsisting by weight of to 22% Cr, 3 to 12% Mo, 0.03 to 0.15% C and atleast one member of the group consisting of 7 to 10% Ta, 6 to 9% Cb and6 to 9% V provided that the total of said group does not exceed about10%, the balance being Ni and residual impurities, provided that thealloy is essentially free of aluminum and titanium and contains not morethan 7% Fe and not more than 8% Co;

(b) solution heat-treating said material at a temperature between 2250and 2300 F. to dissolve at least partially primary monocarbide particlesof the MC type; thereafter (0) quenching the material; thereafter (d)cold-working the material until the desired form is attained; thereafter(e) annealing the material at a temperature in the range of 2250 to 2300F. for a period not exceeding 1 hour; thereafter (f) quenching thematerial to substantially ambient temperature; and thereafter (g) agingthe material within a temperature range of 1100 to 1450 F. until saidtwo co-precipitated coherent phases, A M and MC, are both achieved ineffective strengthening amounts.

7. A method as claimed in claim 6 wherein steps (d), (e) and (f) arerepeated as necessary in order to attain the desired form before agingstep (g) is carried out.

8. A method as claimed in claim 6 wherein the material is aged in step(g) for a period of about 30 to 100 hours.

9. A method as claimed in claim 6 wherein W is substituted for at leastpart of the Mo content specified in step (a).

10. A method as claimed in claim 6 wherein M is principally Ta with someCb associated therewith.

References Cited UNITED STATES PATENTS 3,046,108 7/1962 Eiselstein 1713,069,258 12/1962 Haynes 75-171 3.085,005 4/1963 Michael et al. 75-l713,151,981 10/1964 Smith et al. 75171 3,372,068 3/1968 White 14832.5 X3,466,171 9/1969 Fletcher et al. 75-171 3,497,349 2/1970 Eppich 75-1713,411,899 11/1968 Richard et a1 75171 CHARLES N. LOVELL, PrimaryExaminer US. Cl. X.R.

